To stimulate and/or increase the production of hydrocarbons from a well, a process known as fracturing (colloquially referred to as “fracing”) is performed. In brief summary, a pressurized fluid—often water—is pumped into a producing region of a formation at a pressure sufficient to create fractures in the formation, thereby enabling hydrocarbons to flow from the formation with less impedance. Solid matter, such as sand, ceramic beads, and/or similar particulate-type materials, can be mixed with the fracturing fluid, this material generally remaining within the fractures after the fractures are formed. The solid material, known as proppant, serves to prevent the fractures from closing and/or significantly reducing in size following the fracturing operation, e.g., by “propping” the fractures in an open position. The presence of open, propped fractures provides a beneficial contrast between the permeability in the fracture versus that in the adjacent subterranean reservoir. This contrast provides for a highly conductive pathway through which reservoir fluids may travel, thus enhancing the productivity of the well.
Fracturing using aqueous fluids is often undesirable due to the negative effects of water on formations. For example, clays and other components forming the structure of a formation can swell when exposed to water, while salts and other formation components may dissolve. Accordingly, exposure to a significant quantity of water can destabilize a formation.
Furthermore, use of water and other aqueous fluids also generates issues regarding disposal. Specifically, aqueous fracturing fluid recovered from a well (e.g., subsequent to a fracturing operation) contains various wellbore fluids and other chemicals (e.g., additives to facilitate fracturing using the fluid), and as such, the recovered fracturing fluid must be collected and stored at the surface and disposed of in an environmentally acceptable manner, as required by numerous regulations. Such a process can add considerable time and expense to a fracturing operation, especially when considering the enormous quantities of liquid required to perform such operations.
Non-aqueous fracturing fluids have been used as an alternative to water based ones. One such successful class includes hydrocarbon-based fluids (e.g., crude/refined oils, methanol, diesel, natural-gas condensate, liquefied petroleum gas (LPG) and/or other aliphatic or aromatic compounds). Hydrocarbon-based fracturing fluids are inherently compatible with most reservoir formations, being generally non-damaging to formations while creating acceptable fracture geometry. However, due to the flammability and high vapor pressure of many hydrocarbon-based fluids, enhanced safety preparations and equipment are necessary when using such fluids for wellbore operations. Additionally, many hydrocarbon-based fluids are volatile and/or otherwise unsuitable for use at wellbore temperatures and pressures, while lacking density and/or viscosity sufficient to effectively transport many types of proppant throughout the length of the fracture. As such, it is common practice to use chemical additives (e.g., gelling agents, viscosifiers, etc.) to alter the characteristics of the fluids. An exemplary system describing use of liquid petroleum gas is described in U.S. Pat. No. 8,408,289, which is incorporated by reference herein in its entirety. Use of chemical additives generates waste and disposal issues similar to those encountered when performing fracturing operations using aqueous fluids.
An exemplary hydrocarbon based fracturing fluid would be LPG/Propane, which has a surface tension about ten (10) times less than that of water, and a viscosity about 8 times less than water.
Independent of the type of fracturing fluid and proppant used, a fracturing operation typically requires use of one or more high pressure pumps to pressurize the fracturing fluid that is pumped into a formation, as well as to transport fluid and/or proppant toward the wellbore. The passage of proppant through a pump can damage the pump through an erosive process, especially in the absence of a significant quantity of liquid in the slurry. Great care must often be taken when controlling the proportions of fluid and proppant used to create a slurry to ensure the creation of a mixture that is non-damaging to pumps and other system components, compatible with the formation, is suitable for use within the wellbore (e.g., appropriate specific gravity, stability at wellbore temperatures and pressures, etc.), and to ensure that the proppant is properly transported to and placed in the fractures.
Additionally, pumps and other such equipment are conventionally driven/powered using diesel engines, which can be responsible for significant quantities of noise, emissions, maintenance, and expense at a worksite, and which require the handling of volatile fluids to fuel and operate. Electric drive systems have been contemplated as an alternative to diesel engines; however, such systems require numerous pieces of equipment, extensive cabling and/or similar conduits, and typically utilize on-site power generation, such as a natural gas turbine. Use of turbine prime movers and similar equipment may be unsuitable when utilizing fracturing fluids that include flammable components. An exemplary electrically powered system for use in fracturing underground formations is described in published United States Patent Application 2012/0255734, which is incorporated by reference herein in its entirety.